1. Field of the Invention
This invention relates to the topical application of conjugated linoleic acid together with fatty acid esters of ascorbic acid for the treatment of acute and chronic skin damage. Therapies according to the invention are particularly efficacious for treating a variety of skin conditions including contact dermatitis (particularly diaper area dermatitis), atopic dermatitis, xerosis, eczema, rosacea, seborrhea, psoriasis, thermal and radiation burns, other types of skin inflammation, and the tissue degenerative effects of aging.
2. Description of Related Art
Skin inflammation and aging are closely related phenomena. So similar are the processes involved with both, that aging is sometimes described dermatologically as a chronic low grade inflammatory condition. In acute inflammation, there is typically a respiratory burst of neutrophil activity that initiates cascades that typically involve a change in the oxidation state of the cell. Acute inflammation is also characterized by mast cell degranulation wherein serotonin is produced, which acts as a signal transduction factor. Following that, excited oxygen species are generated, e.g., superoxide anion, and these damage the lipid-rich membranes and activate the chemical mediators of proinflammation and inflammation.
Alteration in the redox state of the cell activates transcription factors such as NF.kappa.B as well as AP1, which then causes production of proinflammation mediators. These mediators, such as TF.alpha. and various interleukins, cause a burst of cytokines. Arachadonic acid is released, which is oxidized to biologically active mediators. When arachadonic acid is oxidized via the cyclooxygenase or lipoxygenase pathways, for example, prostaglandins, leukotrines, and hyroxyeicosatetraenoic acid (HETE) are produced, which cause erythma, edema, and free radical production. Transcription factors such as NF.kappa.B and AP1alter DNA expression in the cell and produce cytokines and proteinases such as collagenase.
Similar metabolic events are observed in skin aging. Cell age is due in part to free radical damage, which takes place mostly within the cell membrane. The cell membrane is most susceptible to attack by free radicals because of its dense molecular structure largely comprising lipids and lipoproteins, which are easily oxidized by reactive oxygen species. In-skin, reactive oxygen species such as singlet oxygen, the superoxide anion, and hydroxyl radicals, as well as other free radicals, are generated in normal metabolism, as well as through ultraviolet sun exposure, other forms of radiation, other environmental factors such as pollution or exposure to chemicals in the home or workplace, and the like, active in the arachidonic acid cascade. As in inflammation, free radicals activate chemical mediators that produce prostaglandins and/or leukotrines.
The body contains an endogenous antioxidant defense system made up of antioxidants such as vitamins C and E, glutathione, and enzymes, e.g., superoxide dismutase. When metabolism increases or the body is subjected to other stress such as infection, extreme exercise, radiation (ionizing and non-ionizing), or chemicals, the endogenous antioxidant systems are overwhelmed, and free radical damage takes place. Over the years, the cell membrane continually receives damage from reactive oxygen species and other free radicals, resulting in cross-linkage or cleavage or proteins and lipoprotins, and oxidation of membrane lipids and lipoproteins. Damage to the cell membrane can result in myriad changes including loss of cell permeability, increased intercellular ionic concentration, and decreased cellular capacity to excrete or detoxify waste products. As the intercellular ionic concentration of potassium increases, colloid density increases and m-RNA and protein synthesis are hampered, resulting in decreased cellular repair. Some cells become so dehydrated they cannot function at all.
In skin aging, the regularity of tissue structure is lost. Individual cells enlarge, but the total number of cells decreases approximately 30%. Intercellular collagen increases, and the proportion of soluble collagen decreases. Cross-linking between long-chain collagen macromolecules occurs. Elastin loses its discrete structure and elasticity, and has an increased calcium content. The dermis microscars and diminishes.
Sunlight and chemical exposure wreaks far greater destruction on the skin than time itself, and intensifies and augments the aging process. There is substantial evidence that ultraviolet radiation induces the formation of reactive oxygen species which are implicated as toxic intermediates in the pathogenesis of photoaging (Ibbotson, S. H., et al., J. Investig. Derm. 112: 933-938 (1999)). Activation of transcription factors such as AP1 causes gene expression of collagenases which cause further damage. Free radical damage to the surface of the skin from sun and chemical exposure is manifested as lines, mottling, discoloration, precancers and cancers. Aging of both skin and other tissues is, in part, the result of constant free radical damage to cell membranes, leading to decreased cell function. This results in accumulation of waste products in the cells, such as lipofuscin; increase in the potassium content of the cells, which results in dehydration of the cells; and decreased production of messenger RNA and proteins.
Early suggestions for dealing with aging effects in skin were predominantly aimed at lubrications and emollients through use of topical compositions containing soothing agents, e.g., as exemplified by commercial hand lotion products and the like. More recently, attention has been directed to agents which address the underlying processes involved in skin damage, such as the free radical generation processes (N.V. Perricone, The Wrinkle Cure, Rodale Press, Emmaus, Pa., 2000). In this regard, investigations have been made with respect to the antioxidants vitamin E and vitamin C to quench free radicals on the surface of the skin and to protect lipid membranes intracellularly (Wilson, R., Drug and Cosmetic Industry, 32-34, 38, and 68, August 1992).
It would be desirable to have alternative topical compositions for skin damage, particularly compositions that are efficient in free radical scavenging in membranes.